It is well known in the art to prepare tetrabromobisphenol-A (TBBPA) by brominating Bisphenol-A (BPA) in various solvents. Methanol is the solvent of choice because it provides excellent productivity. Additionally, TBBPA recrystallizes from the reaction mixture in high yield with high assay, excellent color, low hydrolyseable bromine and low residual ionic bromides. Another advantage is that the use of a methanol solvent allows product collection by simple filtration.
The major disadvantage of employing a methanol solvent in TBBPA processes is that methyl bromide (MeBr) forms as hydrogen bromide (HBr) liberated from the bromination of BPA reacts with methanol. Methyl bromide was once a desirable by-product which could be collected and sold or used in other industrial processes. However, methyl bromide is now a suspect in the depletion of the ozone. Severe restrictions on the sale of methyl bromide are impending. Therefore, it has become imperative to develop cost-effective manufacturing methods which avoid, or at least reduce, methyl bromide co-production, but maintain product quality and yield.
Approaches to reduce the amount of MeBr co-product in methanol solvent processes for making TBBPA have included the use of aqueous HCl or water and lower reaction temperatures. In such processes, the amount of MeBr produced is reduced because the reaction between HBr and methanol is in an equilibrium that is forced to starting material by adding one of the products, water. Lower temperatures slow down the rate at which equilibrium is obtained. However, in a practical cost effective methanol recovery unit, heat is applied during the distillation and water is removed. This leads to the conversion of HBr into MeBr. Hence, these apparent reductions in MeBr are not valid in processes where methanol is recovered by distillation unless the HBr is neutralized before distillation. This is undesirable because it requires base and further processing steps to recover the valuable bromide ion. It is therefore desirable to actually remove HBr from the reaction medium and not simply dilute it with water or reduce the reaction temperature to slow down MeBr production.
Some processes employ hydrogen peroxide to partially or totally replace bromine during TBBPA production. These processes involve the bromination of BPA in a two phase system including water plus an organic phase. These processes do not employ alcohol solvents and hence alkyl bromide co-production is not a concern. These processes are disadvantageous in that they do not give TBBPA in high yield with the good color and low ionics by simple filtration and washing as the methanol process does.
In spite of these efforts, a need has persisted for methods that exploit the excellent properties of methanol for crystallizing TBBPA while reducing MeBr production even when MeOH recovery by simple distillation is employed. A need has also remained for methods that reduce methyl bromide co-production without expensive changes to current protocols and instrumentation for TBBPA production.